1. Field of the Invention
The present invention relates to a casting ladle for use in metal foundries and in particular, but not exclusively, to a ladle for casting aluminium and aluminium alloys, and other non-ferrous metals such as zinc.
2. Description of the Related Art
In aluminium foundries where castings are made using either high pressure die casting or gravity die casting techniques, ladles are normally used for transporting pre-measured quantities of liquid metal from a holding furnace to a casting machine and then pouring the liquid metal into a receptacle of the casting machine. For large scale production processes, the ladle is normally mounted on a mechanical or robotic handling device, which is programmed to dip the ladle into the holding furnace to obtain a measured quantity of liquid metal, and then transport the metal and pour it into the casting machine. For smaller scale production processes, the ladle can be handled manually. The capacity of the ladle is usually quite small (generally between 0.5 and 50 kg of liquid aluminium) and the metal is normally held in the ladle for quite a short time (e.g. less than 60 seconds).
Traditionally, casting ladles have been made of cast iron. This material has the advantage that it can withstand the high temperatures involved and it is very tough. However, it also has the disadvantages that it is attacked by the liquid aluminium and it is very dense (approximately 7000 kg/cm3). Ladles made of this material are therefore very heavy, which causes handling problems and requires the use of powerful handling equipment. Cast iron also has a high thermal conductivity, which causes the liquid metal to lose heat very quickly. The furnace therefore has to be maintained at a temperature that is significantly above the casting temperature to allow for a loss in temperature as the metal is transported, and this results in high energy costs. Casting ladles made of cast iron also have a high maintenance requirement as they must be been cleaned after each casting operation to remove any metal that has solidified and become stuck to the ladle. The ladle must also be coated with a release agent at frequent intervals, for example every one or two days.
In order to reduce some of these difficulties, it is known to coat the casting ladle with a refractory or ceramic coating. However, this is difficult to achieve in practice, since the differential thermal expansion rates of the coating and the underlying cast iron can cause cracking of the coating. Also, most ceramic and refractory coatings are either fragile or wear quickly, and therefore only have a limited lifetime.
It is also known to make casting ladles from cement based refractory materials or from ceramic materials, some of these materials including steel or fiber reinforcements. For example, JP10296427A describes a casting ladle made of a ceramic fiber material, which is reinforced with metal strips and a sheet of a heat resistant woven fabric material. However, ladles made of such materials are generally fragile and/or suffer high wear in use. In addition, there are often problems with attaching ladles made of these materials to mechanical handling devices in such a way that the ladle is adequately supported and does not suffer mechanical failure in use. Such ladles have not therefore gained widespread acceptance.
It is an object of the present invention to provide a casting ladle that mitigates at least some of the aforesaid disadvantages.